The present invention relates to a process for preparing high density solid propellants. More particularly, the present invention relates to a process for preparing high density solid propellants in which hydrogen sensitive salts are employed as the oxidizer. The present invention is particularly useful for the preparation of a solid propellant comprising NF.sub.4 BF.sub.4, high molecular weight polytetrafluorethylene and low molecular weight polytetrafluoroethylene, said propellant having utility as a gas generator for chemical lasers.
High density propellants are typically composed of a fluorocarbon binder, a metal fuel and an inorganic oxidizer salt. Various additives and modifiers well known to those skilled in the propellant art may also be included in the propellant composition. Specific examples of high density solid propellant compositions are set forth in U.S. Pat. Nos. 3,513,043 and 3,876,477.
In general, high density solid propellants are prepared by first dissolving Viton-A, a copolymer of vinylidene fluoride and hexafluoropropylene, in a suitable solvent such as acetone, or another low boiling ketone. Next, the solid ingredients of the composition, namely, polytetrafluoroethylene, metal fuel, oxidizer salt and modifiers, if any, are thoroughly mixed into the Viton-A solution to form a uniform slurry or suspension. Next, a non-solvent for the Viton-A, such as hexane, is added to the suspension. As a result of the addition of the non-solvent the Viton-A precipitates onto the solid ingredients in the slurry. After the suspension settles, the supernatant liquid is decanted and the resulting residue is dried. Once dry, the coated solid propellant ingredients may be subjected to further processing such as extrusion or molding to form propellant grains. The process just described is commonly referred to as the shock-gel or shock-precipitation method for preparing high density composite propellants.
Attempts to prepare high density solid propellants containing hydrogen sensitive oxidizers, such as NF.sub.4.sup.+ salts, according to the shock-precipitation method described above have failed because Viton-A and the processing solvents used therein, are incompatible with NF.sub.4.sup.+ salts.
Experiments have been conducted in an effort to prepare propellants containing hydrogen sensitive oxidizer salts by the cast system in which functionally terminated polymers are reacted with a crosslinking agent to provide a strong binder material. However, it was discovered that oxidizers such as NF.sub.4.sup.+ salts attack all types of hydrogen containing polymeric binders as well as nitroso type binders and binders containing functional groups such as hydroxyl groups, amino groups, and isocyanato groups. In view of this marked incompatibility with such a vast number of organic materials the cast system of curing binders by the use of functionally terminated polymers and crosslinking agents is not suited to the preparation of propellants containing hydrogen sensitive oxidizers, such as NF.sub.4.sup.+ salts.
One method presently being used to prepare propellants containing hydrogen sensitive oxidizers involves mixing the powdered oxidizer salt with powdered polytetrafluoroethylene and pressing the dry mixture into grains in the solid state. However, the propellant grain produced by pressing the mixture of dry components has been found to be unsatisfactory with respect to its homogeneity, grain integrity, and burning rate.
Thus, a need exists for a process for preparing high density solid propellants containing hydrogen sensitive materials which produces propellant grains having superior homogeneity, improved mechanical properties and a more even burning rate as compared to processes presently employed in the art.